Conventionally, urethane foam has been used for heat insulating material used for the heat insulating box of the refrigerator, etc. Recently, according to requests from the market for energy-saving or space-saving and capacity-increasing, instead of the urethane foam, another structure, in which vacuum heat insulating material having heat insulating performance being better than the urethane foam is embedded in the urethane foam and used together, is used. Such vacuum heat insulating material is also used for the refrigerator, etc.
The vacuum heat insulating material is formed by inserting powder, foam, fiber body, etc. as a core material in an outer cover material made of a plastic laminated film, etc. in which aluminum foil is used for a gas barrier layer. Inside of the vacuum heat insulating material, the degree of vacuum is kept to no more than some Pa (pascal).
Further, in order to suppress degradation of the degree of vacuum which becomes a cause of decreasing the heat insulating performance of the vacuum heat insulating material, adsorption agent to sorb gas or water is provided in the outer cover material. For the core material of the vacuum heat insulating material, powder such as silica, foam such as urethane, and fiber body, etc. is used. Currently, glass fiber having excellent heat insulating performance is mainly used for the core material of the vacuum heat insulating material.
Elements of the fiber include inorganic fibers such as glass fiber, ceramic fiber, etc. (refer to Patent Literature 1 and Patent Literature 8, for example).
Further, there are organic fibers such as polypropylene fiber, polylactate fiber, aramid fiber, LCP (liquid crystalline polymer) fiber, polyethylene terephthalate fiber, polyester fiber, polyethylene fiber, cellulose fiber, etc. (refer to Patent Literature 2 and Patent Literature 7, for example).
Shapes of the fiber body include cottonlike, lamination of sheets (refer to Patent Literature 3 and Patent Literature 4, for example), and lamination of sheets with alternating fiber orientations of sheets (refer to Patent Literature 5 and Patent Literature 6, for example).
Further, methods of laminating sheets include lamination by folding a continuous belt-like sheet-shaped member alternatively in different directions so as to form the lamination (for example, refer to Patent Literature 9).